Find out what we're doing about managing flood risks in Lower Hutt.
Lower Hutt is the most densely populated flood plain in New Zealand. The city has a history of flooding, and the projected effects of climate change makes flooding a complex and difficult natural hazard to manage.
To give us a clearer understanding of stormwater flood risks and better prepare for the future, Wellington Water (the precursor to Tiaki Wai) modelled the likely impacts of extreme rainfall on the urban areas of Lower Hutt.
What we're doing to manage flood risk
Council is planning for the forecast impacts of climate change and population growth on our water infrastructure out to the next 30 years.
- council manages flood risk in a variety of ways. The stormwater network of pipes and pumps is designed to drain normal levels of rainfall away from properties, into waterways and on to the coast.
- there are also some retention ponds and wetlands that catch a certain amount of flood water when the system is under pressure.
- historically, storm water networks were designed to carry away water during low to medium intensity rainfall events. When rainfall exceeds the pipe capacity, then water flows overland.
- new buildings are required to include stormwater retention tanks to store water during high rainfall events to delay when stormwater enters rivers and streams.
- The modelling of flood hazards across Lower Hutt will provide crucial data for Hutt City Council for future urban planning to minimise risk, as well as for planning future stormwater and other infrastructure investment.
Identifying at-risk areas of flooding
Computer modelling identifies areas that are affected by heavy rainfall draining from the hills into urban areas, from roofs, roads, footpaths and the water's overland paths to drains, streams and rivers. We convert this information into maps.
The computer modelling:
- is done to best practice standards,
- takes into account the forecasted impacts of climate change out to 2120 and
- is peer reviewed by independent experts.
The resulting stormwater flood hazard maps depict flooding in 1 in 100 year (including allowance for climate change to 2120) and 1 in 10 year floods. They show:
- where waterways are most likely to breach their banks,
- where water moves across land during flooding and
- areas of ponding.
The work represents the most comprehensive flood hazard mapping in the city’s history.
Flood hazard maps
Wellington Water modelled the likely impacts of extreme rainfall on the majority of the urban areas in Lower Hutt.
Flood hazard maps show what flooding looks like for flooding in 1-in-100-year (including allowance for climate change to 2120) and 1-in-10-year floods along with:
- where waterways are most likely to breach their banks
- where water moves across land during flooding, and
- areas of ponding.
How maps help with flooding
The maps will help reduce the impact of flooding by giving council, the community and government agencies a better understanding of future flood events. This means we can improve infrastructure, our urban planning and our responses during extreme rainfall events.
An important part of finalising the maps has been asking local people about their experiences of flooding on their properties and in their neighbourhoods. As part of this process, Wellington Water and council staff undertook public engagement sessions in 2021, showing draft maps to residents and seeking their observations and photos of past events.